The interchange at Interstate 680 and Highway 4 near Martinez and Concord is such a headache that Contra Costa voters in 1988 approved a half-cent sales tax to start planning its fix.

Now, at last, Contra Costa County’s congestion management agency says it has found a path to begin the first phase of the $400 million freeway fix in about two years, pulling it out of an indefinite limbo.

“Voters should be pleased that we have a clear plan to get started on this project,” said Randy Iwasaki, Contra Costa Transportation Authority’s executive director. “If things didn’t change, I can’t tell you how long it would take to get started.”

Under earlier plans, the congestion agency and Caltrans would have waited until the money was lined up to build the most expensive yet effective parts of the five-phase project. Highway planners, however, don’t know when they could afford that.

To break the logjam, the county agency revamped its construction staging and financing plans. The agency plans to start smaller and have more money to spend because of the improving economy. It would begin with widening three miles of Highway 4 to add an extra lane in each direction between Morello Avenue and Highway 242. The widening would cost some $50 million.

The transportation authority also figures it will have $186 million more than previously expected over the next 21 years because of improvements in its financial picture. The agency is taking in more sales tax revenues as the economy recovers. The authority also got an “AA+” credit rating last fall from two rating agencies, enabling it to save millions of dollars in selling $225 million in bonds in December, and refinancing $200 million of existing debt.

With a rosier outlook ahead, the Transportation Authority board on Wednesday is scheduled to authorize consultants to study design on the highway widening. That action could lead to a widening contract being awarded in 2015, Iwasaki said. Adding a third lane in each direction along three miles of Highway 4 will reduce congestion and increase safety for motorists, planners said. One notable commuter says it’s scary to merge from one freeway to another at the interchange.

“It’s a mess,” said Martinez Mayor Rob Schroder, who drives an extra distance to his Walnut Creek business so he can avoid having to merge from one freeway to another. “When my kids were learning to drive, I told them to stay away from this intersection.”

In later phases of the freeway overhaul, contractors will build new connector ramps, remove the cloverleaf connectors, and add a flyover ramp so motorists can stay in a carpool lane continuously while merging from one freeway to another. Getting started on the project makes it easier to seek state and federal grants for later phases of construction, officials said. “You get momentum,” said Susan Miller, the agency’s director of projects.

Each day, some 126,000 vehicles travel on Interstate 680 through the intersection and about 87,000 vehicles travel through it on Highway 4, according to 2011 figures.